The Wide Field InfraRed Survey Telescope (WFIRST) includes a Coronagraph Instrument (CGI) that has baselined a lenslet based IFS to meet imaging and spectroscopy requirements. The current CGI IFS requirements are derived for the shaped pupil coronagraph (SPC) and the hybrid Lyot coronagraph (HLC), which are the two coronagraphs onboard the WFIRST/CGI. However, the WFIRST/CGI instrument must now be compatible with an external occulter (i.e., a starshade) that enables a wider instantaneous spectral bandpass and larger field of view. The detector requirements mandate a photon counting detector, which is only available in a 1024×1024 format, and provide a constraint on the number of detector pixels available to accommodate the new design requirements. Perhaps the only solution to this challenge would be to increase the detector efficiency of the IFS design by making each spatial sampling in the image plane use fewer pixels on the detector.
For an image slicer or any other slit image spectrograph, the crosstalk in the vertical direction is ignored because there is no offset in the spectral dimension. The general approach is shown in the split image spectrograph process 100 of FIG. 1. It is generally accepted that the spectrum versus field has a slow varying effect, or it is considered as spatial resolution. Based on this consideration, the lenslet array can group two spectral traces together to make room for a wider spectrum.
In conventional lenslet array based IFS designs, each lenslet focuses the incident light to a point near the center of the lenslet. In other words, the focus of the near-normal incident beam is at the center of the lenslet. Such a design is shown in lenslet array based IFS 200 of FIG. 2. As can be seen, for each lenslet 210, focus 212 of the normal incident beam is at the center thereof. Naturally, in practice, the array would have more than 4×4 lenslets, and the sixteen lenslets illustrated here are shown to illustrate foci locations.
In order to lay each spectrum without overlap from its neighbor, the spectra are staggered on the detector with spacing to avoid overlap. More specifically, the spectra are staggered such that they are well separated and only partially overlapping, allowing the spectra to be extracted without introducing spectral crosstalk therebetween. For science cases aiming at high dynamic range, such as the WFIRST/CGI IFS, each spectrum may occupy five rows to avoid crosstalk with its neighbors.
The staggering of the spectra may be problematic since one column of the detector will be occupied by different wavelengths of neighboring spectra, where the dynamic range could be quite high (e.g., when there are emission or absorption lines). Another disadvantage of current lenslet array based IFS designs is that the efficiency of detector pixel usage is relatively low. Accordingly, an improved design that improves the efficiency of detector pixel usage may be beneficial.